Gas separation membranes have been in industrial use for around 30 years. Various types of membrane are available, although almost all commercially successful membranes are polymeric membranes formed as flat sheets or hollow fibers.
For use, it is desirable to pack a large membrane area into a small volume. Because membranes are delicate, susceptible to damage and may have a limited operating life, they are normally factory-built into modules or elements.
Two predominant types of membrane modules have emerged. If the membranes are in hollow-fiber form, bundles or hanks of fibers are typically potted in a cylindrical pressure housing or tube. Feed gas may flow on the shell or bore side of the fibers. The permeate or residue gas may be routed to a single collection pipe by which it exits the module.
Flat-sheet membranes are commonly packaged into spiral-wound modules. A spiral-wound module has a central permeate collection pipe, around which are wound multiple membrane envelopes interleaved with spacers to define feed and permeate channels. Feed passes axially down the module across the membrane envelope; permeate spirals inward to the collection pipe.
Modules or elements are normally built in standard sizes. For use, the elements are contained in pressure housings. Each element may be contained within its own housing. Such arrangements are used for hollow-fiber modules, for example, if the number of modules required to perform the separation is modest.
Often the gas mixture being separated includes flammable or hazardous components, so the housing must be made from materials able to safely contain such components. In addition, each housing requires its own piping, valves and connections. As a result, the cost of the housing may far exceed the cost of the membrane element it is designed to hold, making the use of one housing per element very unattractive.
Arrangements that attempt to control costs and reduce the complexity of piping and connections by housing multiple modules within one pressure vessel are in use. For example, individual spiral-wound modules or elements are often housed end-to-end, typically in a line of up to about six modules, within a single pressure-coded stainless or carbon steel tube. Nevertheless, many housings may be required. In natural gas processing, for example, the volume of raw gas being treated can be so large that hundreds or thousands of pressure tubes are used.
The performance of the membrane system is affected by the size and geometry of the membrane elements. For both spiral-wound and hollow-fiber modules, the longer the module, the longer is the flow path for feed or permeate fluids along the membrane surfaces. The result can be a substantial pressure drop from one end of the module to the other on the permeate side, the feed side or both. For example, in a long, shell-side feed hollow-fiber module, the pressure drop on the permeate side along the fiber bores may be as much as 50 psi or more. Such parasitic pressure drops lead to reduced feed-to-permeate pressure ratio and pressure difference, and can adversely affect both the flux and separation capability of the module. Similar effects occur along the leaves of spiral-wound modules. Maintaining acceptably low pressure drop along the feed and permeate channels means that fluid flow path lengths within the modules should preferably be short.
The performance of the membrane system is also affected by the arrangement of the elements in the housing. When elements are connected end-to-end in a tube, the gas to be separated is treated in each module in series. The residue from the first module flows as feed to the second, and so on. Thus, each module in the series experiences a different feed flow and composition, because each subsequent module is treating the residue gas from the adjacent upstream module. If elements can be fed in parallel, then each element will be exposed to the same feed composition and will operate at the same stage cut.
In light of the above issues, there has long been incentive to design configurations by which multiple short elements can be housed within a single housing, and housed in such a way that all of the elements can be fed in parallel.
Over the years various arrangements, many specific to reverse osmosis but some applicable to gas separation, have been proposed for arranging multiple membrane modules or elements within a single pressure housing.
U.S. Pat. No. 3,774,771 describes an assembly of modules mounted in parallel within a single housing. The modules are contained within feed flow tubes, connected so that feed can be introduced to several tubes in parallel, then passed back along the housing through another set of tubes.
U.S. Pat. No. 4,083,780 describes an assembly containing multiple tubes arranged in parallel, with multiple spiral-wound modules mounted in series within each tube.
U.S. Pat. No. 4,451,369 describes an assembly having multiple pairs of hollow fiber modules with a manifolded permeate collection system.
U.S. Pat. No. 4,632,756 describes an assembly in which six hollow fiber elements are mounted within a housing and can be fed in parallel. The assembly includes six permeate tubes for separate withdrawal of permeate from each element.
U.S. Pat. No. 5,071,552 describes an assembly for providing parallel feed to multiple membrane elements in a single housing. The elements are mounted in an end-to-end serial arrangement within the housing.
U.S. Pat. No. 5,238,563 describes an assembly in which multiple membrane modules or elements are housed in parallel. The feed is introduced through a nozzle in the longitudinal shell of the housing and occupies the space between the external surfaces of the modules and the internal surface of the housing.
U.S. Pat. Nos. 7,404,843; 7,510,594; 7,758,670 and 7,918,921, all co-owned with the present application, disclose various configurations for containing gas-separation elements held in an array of tubes within a single pressure housing.
Despite the above innovations, the need for feeding many elements in parallel has not been satisfactorily met. This need is becoming more acute as better, more permeable membranes are being developed.
There remains a need for assemblies that enable many gas separation membrane modules to be housed together in a single pressure housing, and preferably to provide for all of the elements to operate in parallel.